This invention relates to electronic A.C. overload current-responsive electronic circuit breakers and, more particularly, to such a circuit breaker with means for selecting the overload current response level at which the circuit breaker disconnects power from a load.
In many applications where A.C. power is provided to a load, it is necessary to protect the load from overload current in excess of the current-carrying capabilities of the load. Numerous electro-mechanical circuit breakers are known which provide this protection by monitoring the load current and disconnecting power from the load in response to load current exceeding a predetermined overload level. However, due to inherent short-comings such as mechanical lag, potential for mechanical failure and general lack of accuracy, such devices are not always satisfactory, especially where over-current sensitive circuit components may be involved.
While in most applications the current-carrying capability of the load remains fixed, as thus does the overload level to which the circuit breaker is responsive, in some other applications the current-carrying capabilities of the load varies. For instance, the load may comprise a rectifier type power supply which, in turn, may serve a widely varying load that may pose surge and/or short-circuit load hazards to the rectifier elements or other over-current sensitive circuit components of the power supply which may not be absorbed much beyond a one-cycle time interval. Numerous other situations call for a highly sensitive circuit breaker which can be adjusted to respond to different overload current levels.
Known solutions of this problem have included the use of multiple fuses, each responsive to a different current level to remove power from the load. The use of multiple circuit breakers is generally prohibited by cost and space limitations. Known electronic circuit breakers, however, are generally designed to be responsive to only one particular overload current level.